There are known systems for operating electro-luminescent (EL) elements, light emitting diodes (LED), and laser diodes (LD) by means of an inverter circuit for inverting DC power into AC power.
For example, systems for operating EL panels are widely used as back light of liquid-crystal panels as found in compact size liquid-crystal television receivers and lap-top computers having a liquid-crystal display built therein.
In the prior art, different EL operating devices are individually designed in accordance with the dimensions (surface area) of EL elements to be operated and lighting luminance. There exists no operating device which can accommodate singly for varying EL area and lighting luminance.
For long life (LL) inverters which are most popular in the prior art, there is used a system for operating an EL panel so as to follow a resonance frequency which is determined by the C value (electrostatic capacity) of the EL and the L value (inductance) of an inductor. The system is designed to compensate for a lowering of the EL luminance by, in response to a lowering of the luminance of EL due to a lowering of the C value thereof, increasing the resonance frequency and increasing the EL operating frequency in a follow-up manner to the resonance frequency.
However, this method has many problems in that the luminance of lighted EL is naturally determined by the resonance frequency of L and C, and it is difficult to fabricate a device for operating an EL panel having a large surface area. For example, the EL surface area corresponding to A-4 size (400 to 600 cm.sup.2) is currently regarded the practically permissible maximum surface area.
Since it is necessary to switch a high current flow for operating EL panels having a large surface area, the switching loss associated therewith invites an increase of power loss and results in substantial heat release. Thus a heat sink is required as a heat dissipating measure, resulting in a larger size of the operating device.
Additionally, the electric charge accumulated in the EL can be coupled with the charge of the counter electrode, resulting in a power loss and a substantial loss of operating efficacy. For instance, even green color EL panels which are regarded best in operating efficacy show a luminous existence of at most about 1 lumen per watt.